The present invention relates to a method for electroplating a metal strip in a soluble anode system using zinc, tin or other metals as an electrode material.
According to the electroplating method of a steel strip in a soluble anode system, electrodes of a metal for electroplating are arranged in an electrolytic solution in opposition to one or both surfaces of a steel strip. A current is flown using the electrodes as an anode and the steel strip as a cathode, so that the metal of the electrodes may be deposited on the steel strip by electrolysis.
Apparatuses for practicing such electroplating method include those of horizontal type, vertical type and radial type.
In a horizontal type electroplating apparatus, as shown in FIGS. 1A and 1B, a plurality of electrode rows 2, each consisting of a plurality of electrodes arranged horizontally and perpendicularly to the direction of travel of a steel strip 1, are disposed below and above the steel strip 1 travelling horizontally within an electrolytic solution 4. Each electrode row is immersed in the electrolytic solution 4 and is connected to busbars 3.
To a vertical type electroplating apparatus, as shown in FIG. 2, electrode rows 2, each consisting of a plurality of electrodes arranged horizontally and perpendicularly to the direction of travel of the steel strip 1, are arranged at the input side and the output side of a sink roll 6 in opposition to both surfaces of the steel strip 1 which is transferred in a U-shaped form by vertically arranged conductor rolls 5 and the sink roll 6.
In a radial type electroplating apparatus, as shown in FIG. 3, two electrode rows 2, each consisting of a plurality of electrodes arranged perpendicularly to the direction of travel of the steel strip 1 are arranged in opposition to both surfaces of the steel strip 1 which is curved in an arc shape by a conductor roll 7.
In these horizontal type and vertical type electroplating apparatuses, the width of the electrode row 2 is set to be narrower than that of the steel strip 1 by a predetermined amount. This is for the purpose of avoiding the problems to be described below when the width of the electrode row 2 is greater than or excessively smaller than that of the steel strip 1.
When the width of the electrode row 2 is greater than that of the steel strip 1, problems (1) and (2) to be described below occur:
(1) As shown in FIG. 4A, the current from the electrodes 2 is concentrated at the edge portions of the steel strip 1, so that the metal film formed at these edge portions becomes thicker.
(2) As shown in FIG. 4B, since the thickness of only electrodes 8a opposed to the steel strip 1 decreases, it is impossible to keep the gap between the steel strip 1 and the electrodes constant (this is because the electrode rows cannot be brought closer to each other since electrodes 8b at the ends of the electrode row 2 contact each other). When this happens, the voltage must be increased, so that the power consumption increases.
On the other hand, if the width of the electrode row is excessively smaller than that of the steel strip, problem (3) to be described below occurs.
(3) As may be seen from the distribution of the deposition amount shown in FIG. 5, a metal deposited on the portions a little inside of both edges of the strip has a smaller amount than at the central portion of said strip. This results in irregular distribution of the deposition amount along the direction of width of the strip.
For the reasons (1), (2) and (3) described above, the width of the electrode row is conventionally adjusted according to changes in the strip width. According to the method for this adjustment, as the strip width decreases, the electrodes at the ends of the electrode rows are unloaded. However, this adjustment method presents following problems (4) to (7):
(4) The lower electrode row of the horizontal type apparatus and the electrode rows of the vertical type apparatus are respectively arranged below the steel strip and the conductor roll. Therefore, the accessibility for unloading the electrodes at the ends of the electrode rows for the purpose of decreasing the width of the electrode row is poor.
(5) The thickness of the individually unloaded electrodes is not so small as to justify disposal but is not uniform. If these electrodes are disposed, the use efficiency of the electrodes is degraded. On the other hand, if these electrodes are to be put to use again, they must first be stored in great quantity and must then be grouped into electrode rows of substantially the same thickness.
(6) As may be seen from the graph shown in FIG. 6, even if the width (line s) of the steel strip decreases linearly, the width (stepped line e) of the electrode row decreases in a stepped manner. Therefore, the difference between the width of the electrode row and that of the steel strip becomes maximum when the electrodes at the ends of the electrode row are unloaded. Then, the width of the electrode row becomes too small as compared with the strip width. This results in the nonuniformity of the deposition amount of the metal as shown in FIG. 5. In order to prevent this, the width of each electrode constituting the electrode row may be decreased. However, this results in a greater frequency of unloading of electrodes, which is not preferable.
(7) In the horizontal type apparatus, as shown in FIG. 7, the busbar 3 for energizing the electrode row 2 arranged below the steel strip 1 is in direct opposition with the steel strip 1 in the electrolytic solution. Therefore, the current flows from the busbar 3 to the steel strip 1, and the busbar 3 is electrolytically corroded. This electrolytic corrosion of the busbar 3 is notable when a chloride bath is used as an electrolytic solution.
Problems (4) to (7) described above may be solved by increasing the width of the electrode row in excess of the strip width. However, when this measure is taken, problems (1) and (2) as described above occur. In order to solve problem (1), a method is developed according to which an edge mask is arranged in the vicinity of the edge of the steel strip 1 in order to avoid the current concentration at the strip edge. However, even when this measure is taken, problem (2) still remain unsolved.
In order to solve problem (2), the electrode transfer method is known which is conventionally adopted in tin plating. According to this method of electroplating, as shown in FIGS. 8A and 8B, electrodes 8 of sequentially varied thicknesses are arranged on inclined busbars 3, so that a constant gap is kept between the steel strip 1 and the respective electrode 8. When the thickness of each electrode is decreased by a thickness corresponding to the thickness difference between the adjacent electrodes, the electrode row 2 is displaced in the direction indicated by the arrow for a distance corresponding to the width of one electrode. Then, the electrode of least thickness is unloaded from the left in the direction indicated by the arrow, and a new electrode is loaded from the right. According to this method, the gap between the electrodes 8 and the steel plate 1 may be kept constant. However, if the width of the electrode row 2 is smaller than the width of the steel strip 1, problems (4) to (7) with the conventional adjustment method cannot be solved. This method especially suffers from the fatal disadvantage of low use efficiency of the electrodes.
Thickness t.sub.w (in mm) of the electrode unloaded for treating a steel strip of a given width W (in mm) is given as: EQU t.sub.w =T-W(T-t)/Wmax
where T is the thickness (in mm) of an electrode which is loaded anew; t is the width (in mm) of the electrode which is unloaded when the width of the steel strip is Wmax; and Wmax is the maximum width in mm of the steel strip used in the treatment line.
The use efficiency .alpha..sub.w of the electrode is given as: EQU .alpha..sub.w =(T-t.sub.w)/T=(W/Wmax)(T-t)/T
(T-t)/T corresponds to the use efficiency of the electrodes when a steel strip of the maximum thickness is used. (T-t)/T is thus the maximum use efficiency .alpha.max. Therefore, EQU .alpha..sub.w =W/Wmax.multidot..alpha.max
On the other hand, the minimum use efficiency .alpha.min is given as: EQU .alpha.min=Wmin/Wmax.multidot..alpha.max
where Wmin is the minimum width of the steel strip to be used in the treatment line.
In the case of tin plating wherein there is only a small difference between the maximum width and the minimum width of the strip, the minimum use efficiency does not become very low. However, in the case of zinc plating of a steel plate having a maximum width of 1,819 to 1,219 mm and a minimum width of 900 to 610 mm, the minimum use efficiency decreases to 1/2 to 1/3 the maximum use efficiency. According to the electrode transfer method described above, the unloaded electrode of greatest thickness is smaller than the thickness of the electrode which is loaded anew, the used electrodes may not be used again and all of them must be disposed of. This results in a low use efficiency.
As an improvement over the method shown in FIGS. 8A and 8B, a method is proposed which is adopted in the radial type apparatus. According to this method, as shown in FIG. 9, the width of the electrode row 2 is made greater than the strip width and the edge mask 9 is used. Although problems (4) to (7) of the conventional adjustment method are solved, problem (5), that is, the decrease in the use efficiency of the electrodes, and the fact that the electrodes cannot be used again, is not solved. Furthermore, as shown in FIG. 9, the electrodes 8 which are not opposed to the steel strip 1 are in the stepped form. Therefore, it is impossible to arrange the edge masks 9 as shown in FIG. 9 and then to displace them to the right or left in accordance with the shift of the steel strip 1.